With increasing awareness about environment problems in recent years, greater attention is focused on solar batteries as a clean energy source. Development activities and proposals have been made for various types of solar cell modules. In general, solar cell modules contains a photovoltaic cell such as crystalline silicon photovoltaic cell and amorphous silicon photovoltaic cell, and they are produced by laminating a surface protection sheet, a encapsulant sheet of, for instance, ethylene-vinyl acetate copolymer resin, a photovoltaic cell, encapsulant sheet, and a back face protection sheet layer, stacked in that order and integrated by vacuum aspiration and thermocompression bonding. Back face protection sheets used in solar cell modules are generally produced from a plastic base material that is lightweight and high in electric characteristics and strength.
Solar batteries are required to maintain high performance for a long term of 20 years or more. Back face protection sheets used in solar cell modules must have high strength, weather resistance, heat resistance, water resistance, light resistance, chemical resistance, light reflectivity, light diffusion properties, moisture resistance, antifouling properties, design properties, etc., which are required to be maintained for a long period without deterioration.
For solar cell modules, back face protection sheets containing a polyester film, such as polyethylene terephthalate, which has high weather resistance and electrical insulating properties, have been developed. Techniques for improving weather resistance and hydrolysis resistance, which polyester film inherently lacks, disclosed so far include using a material containing a ultraviolet absorber (Japanese Unexamined Patent Publication (Kokai) No. 2001-111073), specifying a content of a cyclic oligomer in a polyester film (Japanese Unexamined Patent Publication (Kokai) Nos. 2002-100788 and 2002-134771), and specifying a molecular weight of a polyester film (Japanese Unexamined Patent Publication (Kokai) No. 2002-26354).
When using such a polyester film in combination with other materials to form a layered body, it is important to select a suitable adhesive for their bonding, and the known suitable ones include polyurethane based adhesives containing a carbodiimide compound, oxazoline compound, or epoxy compound (Japanese Unexamined Patent Publication (Kokai) Nos. 2008-004691 and 2007-320218).
There is nonetheless a need to provide a back face protection sheet for solar cell modules that does not suffer from significant deterioration in adhesive strength over a long term use under high-temperature, high-humidity conditions.